NARRATOR: He warned us, more than 25 years ago, that human activity was changing the Earth'sclimate. Since then, the world has gotten hotter, and NASA scientist James Hansen's warning has been echoed by the vast majority of climate scientists everywhere.

JAMES HANSEN (NASA Goddard Institute for Space Studies): Global warming in the past century is about eight-tenths of a degree Celsius, with most of it occurring in the last 30 years.

NARRATOR: And now the warnings have become more urgent.

JAMES HANSEN: I don't agree that we've passed the point where there's no hope, but, but, on the other hand, we're darned close.

NARRATOR: Close, because scientists have uncovered a new factor that may be masking the full impact of global warming. Called global dimming, it's powerful enough to alter temperatures in a matter of days. It may have contributed to the world's deadliest drought, and it could mean that the Earth's climate is about to start heating up as fast as the most dire predictions.

JAMES HANSEN: I think we have less than a decade to avoid passing what I call "point of no return."

NARRATOR: What will the future of our planet be, now that we're Dimming the Sun? Right now on NOVA.

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The show starts here:
NARRATOR: September 12th, 2001, the aftermath of tragedy: ironically, as America mourned, the weather all over the country was unusually clear and sunny. Eight hundred miles west of New York, in Madison, Wisconsin, climate scientist David Travis was on his way to work.

DOCTOR DAVID TRAVIS (University of Wisconsin-Whitewater): Around the 12th, later on in the day, when I was driving to work, and I noticed how bright blue and clear the sky was, and...at first I didn't think about it, then I realized the sky was unusually clear.

NARRATOR: For 15 years, Travis had been researching a relatively obscure topic: whether the vapor trails left by aircraft were having a significant effect on the weather. In the aftermath of 9/11, the entire U.S. fleet was grounded, and Travis finally had a chance to find out.

DAVID TRAVIS: It was certainly, you know, one of the tiny positives that may have come out of this—an opportunity to do research—that hopefully will never happen again.

NARRATOR: Travis suspected the grounding might make a small, but detectable, change to the weather, but what he observed was both immediate and dramatic.

DAVID TRAVIS: We found that the change in temperature range during those three days was just over one degree centigrade. And you have to realize that from a layman's perspective that doesn't sound like much, but from a climate perspective that is huge.

NARRATOR: The temperature range is the difference between the highest and the lowest temperatures in a 24-hour period. Usually, it stays much the same from day to day, even if the weather changes, but not this time. Travis had come across a new and powerful phenomenon, one which would call into question all our predictions about the future of our planet.

The trail that would lead to this extraordinary discovery of global dimming began 40 years ago, in Israel, with the work of Gerry Stanhill, a young English immigrant. Trained as a biologist, Gerry got a job helping to design irrigation systems. His task was to measure how strongly the sun shone over Israel.

DOCTOR GERALD STANHILL (Israel Ministry of Agriculture): It was important, for this work, to measure solar radiation, because that is the factor that basically determines how much water crops require.

NARRATOR: For a year, Gerry collected data from a network of light meters. The results were much as expected and were used to help design the national irrigation system. But, 20 years later, in the 1980s, Gerry decided to update his measurements. What he found stunned him. ( Continued at